The present invention relates generally to illumination optical systems, and more particularly to an illumination optical system used to manufacture devices, such as single crystal plates for semiconductor wafers and glass plates for liquid crystal displays (“LCD”). The present invention is suitable, for example, for an illumination optical system for projection exposure to an object with a contact-hole line pattern or a mixture of isolated contact hole and contact-hole line in a photolithography process.
Recent demands on smaller and thinner profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. For example, a design rule has attempted to form a circuit pattern of 100 nm or less on a mass production line, and which will expectedly shift to 80 nm or less. The mainstream photolithography technology has conventionally used a projection exposure apparatus that projects and transfers a pattern on a mask (a reticle) onto a wafer.
Rayleigh equation provides the resolution R of the projection exposure apparatus using a light-source wavelength λ and a numerical aperture (NA) of the projection optical system as follows:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
A focus range that may maintain certain imaging performance is called a depth of focus (DOF), which is defined in the following equation:                     DOF        =                              k            2                    ×                      λ                          NA              2                                                          (        2        )            
Since small DOF makes focusing difficult and thus requires strict flatness and focus accuracy, larger DOF is preferable.
A mask pattern includes an adjacent and periodic line and space (L & S) pattern, an adjacent and periodic contact-hole line that arranges holes at an approximately hole interval, an isolated contact hole that does not have a pair and thus is isolated, other isolated patterns, etc. A pattern transfer with high resolution requires a selection of optimal illumination condition in accordance with pattern types.
The recent semiconductor industry has shifted its production to a highly value-added system chip that mixes a wide variety of patterns. However, the prior art cannot finish exposure such a contact-hole pattern at one time with high resolution, as blends a contact-hole line and an isolated contact hole.
Various methods have been proposed to increase DOF by improving the resolution limit only for a contact-hole line and a longitudinally and laterally periodic wire pattern. These methods include, for example, a double exposure or multi-exposure method that uses two masks to separately expose different types of patterns, an exposure method that uses one mask and special illumination conditions, and a method that provides a mask with various auxiliary patterns to improve the resolving power for a desired pattern.
The above methods commonly require an illumination optical system that serves to freely vary illumination conditions, specifically an effective light-source distribution of the illumination optical system, whenever a size and arrangement of a mask pattern changes according to processes. Disadvantageously, the conventional illumination optical system cannot provide this function, or obtain a high resolution with an optimal illumination condition.
Prior art disclose a switch mechanism from a usual circular effective light source to an annular effective light source, and a switch mechanism to a quadrupole effective light source. Nevertheless, an effective light source needs to vary even in the same type to improve the resolution for future finer patterns.